This invention is in the field of light emitting semiconductor devices. Light emitting diodes ("LED"s) with highly reflective contacts and methods for making them are the particular subject of this disclosure.
In known LEDs, the extraction efficiency, which is a ratio of the amount of light leaving the finished LED to the amount of light actually generated by the LED, is determined by two competing processes: internal scattering of light into light escape cones within the LED and internal absorption of light within the bulk of the LED or at lossy surfaces.
Due to their high density of free charge carriers, the contacts which allow a voltage to be applied across an LED absorb a great deal of the light generated by the LED. Minimizing the size of the LED's contacts increases the extraction efficiency of the LED, as long as sufficient ohmic surface area is retained. Further, if the contacts can be made reflective, internal reflection of light is increased. This improves the extraction efficiency, as the reflected light will eventually scatter into one of the LED's light escape cones if the absorption coefficient within the LED can be reduced sufficiently to permit 10-100 internal reflections within the LED chip.
Reflecting contacts have been known since the 1970's. Two approaches, dielectric mirrors with contact holes and shadow mask evaporation, are used to fabricate these reflecting contacts. To form a dielectric mirror with contact holes, a SiO.sub.2 layer is deposited on the back surface of the LED wafer and small holes with a diameter of 15-25 .mu.m are etched through the SiO.sub.2 layer using photolithography. As the wafers from which the LED chips are fabricated are generally not completely flat, optical resolution with conventional photolithographic techniques limit the hole size to no less than 10-15 .mu.m in high volume production. Consequently, the contacts typically cover 25% of the LED's back surface, resulting in an area weighted reflectivity of 70-75%. After being reflected twice by the rear contacts, a photon has a 50% chance of being absorbed. FIG. 2 shows an LED using this type of known reflective contact.
To avoid the cost of the SiO.sub.2 deposition and subsequent photolithographic steps, a contact metal can be evaporated through a shadow mask to form contacts. The resulting contacts still cover roughly 25% of the LED's back surface and perform similarly to those formed using the dielectric mirror with contact holes.
As both these known techniques offer only marginal improvements in light extraction efficiency, simpler metal contacts covering the entire back surface of the LED are often used.
LEDs with highly reflective but still conductive back contacts and methods for fabricating them are therefore desirable.